$cookie = Mage::getSingleton('core/cookie');
if ($key = $cookie->get('secret_key')) { // user has cookie, retrieve data
   $collection = Mage::getModel('[namespace]_[module]/values')->getCollection()
      ->setFieldToFilter('key', $key)
      ->setFieldToFilter('ip', Mage::helper('core/http')->getRemoteAddr(true));

   $value = $collection->getFirstItem()->getData('value');

} else { // new user so set data
   $value = 'The Value You Want To Store';
   $key = uniqid();

   // save key for reference
   $cookie->set('secret_key', $key);

   // store in database
   Mage::getModel('[namespace]_[module]/values')->setData(array(
      'key' => $key,
      'ip' => Mage::helper('core/http')->getRemoteAddr(true),
      'value' => $value,
   ))->save();
}